The invention relates generally to subsurface drilling operations and particularly to techniques for predicting pore pressures ahead of a drill bit while drilling a borehole through subsurface formations.
A column of drilling fluid, usually referred to as “mud,” is customarily provided in a borehole while drilling the borehole through subsurface formations. Usually, the weight of the mud is carefully selected such that the hydrostatic pressure gradient in at least the uncased section of the borehole is above the pore pressure gradient and below the fracture pressure gradient in the surrounding subsurface formations. If hydrostatic pressure gradient is lower than pore pressure gradient, a kick or blowout may occur. If hydrostatic pressure gradient is higher than fracture pressure gradient, lost circulation may occur. Fracture pressure typically increases rapidly with depth so that maintaining hydrostatic pressure gradient below fracture pressure gradient after drilling an initial section of the borehole is usually less of a problem. Pore pressure on the other hand generally follows a less predictable pattern. To avoid drilling hazards, it is desirable to know the pore pressure gradient ahead of the drill bit so that the mud weight needed to provide the desired hydrostatic pressure gradient in the borehole can be determined prior to drilling an interval ahead of the drill bit.
Methods are known in the art for predicting pore pressures ahead of a drill bit using well log data and/or seismic survey data. One common method of predicting pore pressures ahead of a drill bit using well log data involves determining a normal compaction curve from a well log, e.g., a sonic log, in combination with an appropriate geological model. The normal compaction curve corresponds to the increase in formation density that would be expected as a function of depth assuming absence of abnormal pressure. During drilling, a logging-while-drilling (LWD) sonic log is obtained and compared to the normal compaction curve. A consistent slowing trend of the LWD sonic log away from the normal compaction curve is used as a likely indicator of increased pore pressures ahead of the drill bit. This expected increase in pore pressures can be estimated from the amount of departure of the LWD sonic log from the normal compaction curve. The accuracy of this method largely depends on the accuracy of the well log data used in generating the normal compaction curve. Since this method relies on well logs, the well has to be drilled into the over-pressured zone for it to be detected.
U.S. Pat. No. 5,130,949 (issued to Kan et al.) discloses a method of predicting pore pressures ahead of a drill bit using well log data and surface seismic data. The method involves using well log data to estimate a shale fraction for a subsurface formation as a function of depth and the derivation of a shale compaction trend as a function of depth at intervals where the estimated shale fraction exceeds a threshold. A translation curve expressing pore pressure gradient as a function of sonic interval transit time departure from the shale compaction trend is also derived using the well log data. Seismic observations are performed along a surface line, and interval transit times as a function of depth along the surface line are estimated from the seismic observations. The departures of the seismic interval transit times from the shale compaction trend as a function of depth for points along the surface line are computed. The departures are translated into pore pressure gradient predictions using the translation curve. The patent suggests improving pore pressure predictions in deviated wells by adjusting interval transit time data for the shallower portion using check-shot data and applying the adjustments to an entire seismic section to obtain better depth.
U.S. Pat. No. 5,144,589 (issued to Hardage) discloses a method of estimating pore pressures ahead of a drill bit using drill-noise seismic. During drilling, the noise of the drill bit as it impacts the earth is used as a seismic source. Some seismic signals propagate directly from the drill bit to the surface. Some seismic signals propagate downwardly and are reflected back to the surface. The direct signals are used to determine interval velocity for each formation through which the drill bit has drilled, and the interval velocity is continuously updated as the drill bit penetrates the earth. The reflected signals are used to determine acoustic impedance for each formation ahead of the drill bit. The interval velocity data and acoustic impedance information are combined to produce a log-like impedance estimation curve, which reflects the pore pressures ahead of the drill bit. In particular, the low frequency velocity trend immediately above the drill bit is extrapolated to produce the low frequency velocity behavior for a short distance, e.g., 100 to 500 feet, below the drill bit. This low frequency behavior is then used to correct the acoustic impedance data for the formation ahead of the drill bit. Drill-noise seismic is generally inefficient in soft sediment, highly deviated boreholes, and while drilling with certain types of bits, such as polycrystalline diamond compact bits. Furthermore, estimation of acoustic impedance ahead of the bit from reflection data rarely works and is not reliable.
A new technique called seismic measurements-while-drilling (SMWD) has been applied to pore-pressure prediction ahead of a drill bit. In one implementation, a pore-pressure map is generated using surface seismic data. To obtain the map, velocities are estimated from pre-stack surface seismic data, and a velocity-to-pore-pressure transform appropriate for the area is used to convert the velocities to pore pressures. The pore-pressure map has the coordinates of the seismic shot position (horizontally) and seismic travel time (vertically). During drilling, SMWD is used to deliver real-time check-shot data. The check-shot data includes travel times of seismic waves generated at the surface as a function of depth. The check-shot data are used to place the drill bit on the pore pressure map, thereby allowing the pore pressures ahead of the drill bit to be determined. The accuracy of this method depends on the accuracy of the pore-pressure map predicted from the surface seismic data. In general, the accuracy of the pore-pressure map predicted in this manner decreases as depth increases because the velocities estimated from the surface seismic data become less accurate with increasing depth.
Thus a need remains for more robust techniques for estimating pore pressures ahead of a drill bit.